WARSAW: Staff at the zoo in the Polish town of Plock first noticed the sound, not the sight, of two Siberian tiger cubs born in late April, keeper Malgorzata Trzcinska said, introducing the first of the breed born in the country this year.

"The mother was very careful and very effectively hid them from our sight," Trzcinska said this week, adding that it was the cubs' distinctive meowing that gave them away.

The breed is part of the European Endangered Species Programme, making the cubs' birth that much more "exciting," Trzcinska said. Only 23 Siberian tigers have been born in Europe in the last 12 months.

"A female Siberian tiger can give birth to up to six cubs ... but we are very pleased with our pair. We have a male and a female so we think this is good for the future of the breed."

While the cubs are now big enough to fight playfully with each other and the brother is already larger than his sister, the cubs have yet to be named.

"We have announced a competition ... that will help come up with a name for our kittens," Trzcinska said. "It's a free choice ... but we have certain criteria that will help us decide."